In recent years, silica is blended and used so frequently as a filler which causes low heat generating property of a rubber composition for tires to be consistent with a gripping property of a tire on a wet road.
A low heat generating property of the rubber composition for tires leads to fuel consumption saving effect of automobiles, and gripping property of a tire on a wet road provides a good effect to safety of a tire.
In this connection, in order to further improving a gripping property of a tire, it is effective to blend silica for capturing a road surface to rubber components in a high content. Further, increasing a particle diameter of silica allows irregularities on a road brought into contact with a tire to be captured more surely by silica particles on the surface of the tire, whereby the tire is further increased in gripping property.
However, silica used tends to cause coagulation of particles themselves by virtue of a hydrogen bond of a silanol group which is a surface functional group, and in order to improve dispersibility of silica in a rubber, kneading time has to be elongated. Also, silica has the defect that Mooney viscosity of a rubber composition for tires is raised due to an unsatisfactory dispersibility of silica in a rubber and that the rubber composition is inferior in processability such as extrusion.
The problems described above are markedly brought about when silica is added in a higher content, and they are a large barrier in increasing a blend amount of silica in a rubber composition for tires.
Accordingly, it is required to improve processability and the like in a rubber composition for tires which is blended with silica in a high content in order to cause the tire to exhibit high gripping property on wet roads and the like.
There have so far been known as a technology for improving a processability and the like in a silica-blended rubber composition for tires prepared by using glycerin fatty acid esters, for example:
1) a rubber composition improved in an electrostatic property which is prepared by blending 100 parts by mass of a rubber containing 90 parts by mass or more of a diene base rubber with 30 to 120 parts by mass of a filler containing 40% by mass or more of a white filler, and 0.2 to 8 parts by mass of a nonionic surfactant (refer to, for example, a patent document 1), and2) a rubber composition for a tire tread containing at least one polymer selected from the group of diene base rubbers, and 5 to 100 parts by mass of fine powder non-precipitated silicic acid, 0 to 80 parts by mass of carbon black, and 0.5 to 20 parts by mass of at least one non-aromatic viscosity-reducing substance each based on 100 parts by mass of a rubber contained in the rubber composition, wherein the non-aromatic viscosity-reducing substance described above is at least one substance selected from the group consisting of glycerin monostearate, sorbitan monostearate, sorbitan monooleate, and trimethylolpropane(2-ethyl-2-hydroxymethyl-1,3-propanediol) (refer to, for example, a patent document 2).
On the other hand, known as a technology for improving dispersibility of silica in a rubber, processability and the like in a silica-blended rubber composition prepared by using compounds other than the glycerin fatty acid esters described above are, for example:
3) a tire characterized by that a rubber composition prepared by blending at least one rubber component selected from a natural rubber and/or a diene base synthetic rubber with hydrous silicate which has fine pores and in which an ink bottle-shaped pore index (IB) determined by a nitrogen adsorption specific surface area (BET) (m2/g) and prescribed measurement and calculation satisfies a specific formula, and at least one compound selected from the group consisting of specific amide and amine compounds is used for tire members (refer to, for example, a patent document 3), and4) a composition prepared by blending 100 parts by mass of a rubber component comprising a natural rubber and/or a diene base synthetic rubber with 15 to 85 parts by mass of silica and 1 to 15% by mass of a specific tertiary amine compound such as dimethylalkylamine and the like based on the amount of the silica described above, and a pneumatic tire prepared by using the above composition for a tire tread (refer to, for example, a patent document 4), and5) a rubber composition prepared by blending a rubber component comprising a natural rubber and/or a diene base rubber with a white filler and at least one specific monoalkanolamide, and a tire prepared by using the above composition (refer to, for example, a patent document 5).
A rubber composition in which glycerin fatty acid monoester is blended together with silica in one of the examples is described in the patent document 1 out of the patent documents 1 to 5, and it is described therein that an effect of preventing electrification which is different from that of the present invention is obtained and that the effect is valid as well for a tire tread part abraded by friction. However, it is neither described nor suggested therein that a viscosity-reducing effect is obtained and that a tire prepared by blending a silica-blended rubber composition blended with glycerin fatty acid monoester is inhibited from being reduced in abrasion resistance.
Also, viscosity-reducing effect observed when the tire is blended with glycerin fatty acid monoester together with silica is described in the foregoing patent document 2, but it is neither described nor suggested therein whether or not a tire prepared by blending a silica-blended rubber composition blended with glycerin fatty acid monoester allows an abrasion resistance to be consistent with viscosity-reducing effect.
It is merely described in the patent document 3 that silica is enhanced in dispersion by using the specific compound, and a glycerin fatty acid monoester composition is not described therein. Also, it is described in the patent document concerned that silica having a large particle diameter is used, but blending the silica having a large particle diameter in a selected high ratio, and the effects thereof are neither described nor suggested therein.
Further, the rubber compositions prepared in the patent documents 4 and 5 described above are improved in dispersibility of silica in a rubber and a heat generating property to such an extent as has not so far been observed. In the patent documents 4 and 5, however, influences of the silica-blended rubber composition on an abrasion resistance of the tire is neither described nor suggested therein.
Further, the rubber compositions prepared in the patent documents 4 and 5 described above are reduced slightly in processability due to shrinkage. Also, the problem that the scorch time is expedited to cause rubber yellowing is brought about in the patent document 4.
In all of the patent documents 3 to 5, the nitrogen base compounds such as amine, amide and the like are used to disperse silica, but in the present invention, the glycerin fatty acid monoester composition which is different from the above nitrogen base compounds is used.